1. Field of the Invention
The present invention relates to computer display devices that display graphics and more particulary to a system for expanding the available graphics memory. The invention is particularly suited for increasing the graphics resolution produced by computer display device.
2. Description of the Prior Art
Various computer display devices are well known which include a screen for displaying graphics. Such devices include raster scan devices wherein the display device screen displays a plurality of individually addressable picture elements (pixels) arranged in a matrix. Graphics memory within the display device includes one or more memory bits for each pixel with the visual representation of the pixel controlled by the state of the associated bit or bits. For example, where one bit per pixel is available, the state of the bit (that is, "on" (one) or "off" (zero) in a binary system) defines the visual representation of that particular pixel on the display device screen.
Generally, a dedicated graphics controller controls the visual representation shown by the display device screen. The graphics controller can be part of the display device or can be part of a computer system. For example, the graphics controller can be "plug-compatible" with the particular address, data and control buses of the computer system, in which case the graphics controller comprises one or more plug-in system cards. The graphics controller can include circuitry to control the timing necessary to synchronize the display device and the memory for the graphics display, thereby determining the maximum graphics display resolution.
It is often desirable to vary the resolution of the graphics display, that is, the number of pixels comprising the graphics matrix. This may occur where a low resolution graphics display is first purchased because of lower cost but which subsequentally becomes inadequate to meet the needs of the user. However, in order to provide graphics resolution greater than the maximum resolution available from the graphics memory of the controller, the controller must be replaced with a new controller having expanded graphics memory capabilities. Since the entire graphics controller must be replaced, the cost of increasing the graphics resolution is relatively great. This high cost consequently can discourage a graphics display user from upgrading the graphics resolution despite its desirability where, for example, complex graphic representations are to be displayed on the display device screen.
Thus, there is a need for a graphics display system wherein the resolution of the graphics display can be easily and inexpensively changed.